It is known that aromatic isocyanates can be converted into primary aromatic amines by acid hydrolysis (for a fairly recent example, see Japanese Pat. No. 55 007-827). However, the reaction is incomplete by this method because the amine formed during hydrolysis further reacts with unreacted isocyanate to form the corresponding urea. This further reaction cannot be suppressed even by using excess strong mineral acid.
German Auslegeschrift No. 1,270,046 describes a process for the production of certain primary aromatic amines containing polyalkylene glycol ether segments in which reaction products of aromatic diisocyanates or triisocyanates with polyalkylene glycol ethers and/or polyalkylene glycol thioethers (preferably those having molecular weights in the range of from 400 to 4000) are reacted with secondary or tertiary carbinols. The products of this reaction are then subjected to thermal dissociation at elevated temperatures in an inert solvent (optionally in the presence of acid catalysts). Apart from the high dissociation temperature, the disadvantage of this process lies in the fact that flammable, readily-volatile alkenes (which are explosive in admixture with air) are formed during the thermal dissociation of the urethanes, necessitating that appropriate safety measures to be taken.
German Auslegeschrift No. 1,694,152 relates to the production of prepolymers containing at least two terminal amino groups by reacting hydrazine, aminophenyl ethylamine or other diamines with an NCO-prepolymer of a polyether polyol and polyisocyanates (in an NCO:NH ratio of from 1:1.5 to 1:5). In this process, unreacted amine has to be carefully removed in another process step because it catalyzes the reaction with polyisocyanates to a considerable extent, thus leading to short processing times in addition to actually participating itself as a reactant.
Another possible method for synthesizing polyamines containing urethane groups is described in French Pat. No. 1,415,317. NCO-prepolymers containing urethane groups are converted with formic acid into the N-formyl derivatives which are hydrolyzed to form terminal aromatic amines. The reaction of NCO-prepolymers with sulfamic acid according to German Auslegeschrift No. 1,155,907 also leads to compounds containing terminal amino groups. In addition, relatively high molecular weight preadducts containing aliphatic, secondary and primary amino groups are obtained in accordance with Auslegeschrift No. 1,215,373 by reacting relatively high molecular weight hydroxyl compounds with ammonia in the presence of catalysts under pressure and at elevated temperature or, in accordance with U.S. Pat. No. 3,044,989, by reacting relatively high molecular weight polyhydroxyl compounds with acrylonitrile, followed by catalytic hydrogenation. According to German Offenlegunsschrift No. 2,546,536 and U.S. Pat. No. 3,865,791, relatively high molecular weight compounds containing terminal amino groups and urethane groups are also obtained by reacting NCO-prepolymers with enamines, aldimines or ketimines containing hydroxyl groups, followed by hydrolysis of the resulting compounds.
Still another possible method for synthesizing aromatic polyamines containing urethane and ether groups lies in the ring-opening process which takes place during the reaction of isatoic acid anhydride and diols. Polyamines of the type are described, for example, in U.S. Pat. No. 4,180,644 and in German Offenlegungsschrift Nos. 2,019,432; 2,619,840; 2,648,774 and 2,648,825. The resulting poor reactivity of aromatic ester amines obtained in this way, however, is a disadvantage for numerous applications.
The reaction of nitroaryl isocyanates with polyols, followed by reduction of the nitro groups to aromatic amine groups, is also known (U.S. Pat. No. 2,884,439). The primary disadvantage of this process lies in the high cost of the reduction step.
It is also known that certain heteroaromatic isocyanic acid esters can be converted into heteroaromatic amines by basic hydrolysis. However, the hydrolysis conditions described for two quite specific heteroaromatic monoisocyanic acid esters in H. John J. Prakt. Chemie, 130,314 et seq and 332 et seq (1931) are not only totally unsuitable for the conversion of polyisocyanate compounds into aliphatic and/or aromatic amines, they are also dangerous.
Two of Applicants' own earlier applications Nos. (P 29 48 419.3 and P 30 39 600.0) described multistage processes for the production of polyamines by the alkaline hydrolysis of NCO-preadducts, with excess quantities of a "strong" base (aqueous solutions of alkali- and/or earthalkali hydroxides or tetraalkylammonium hydroxides) to form carbamates; followed by acidification with mineral acids or ion exchange resins in quantities exceeding the quantity of base, resulting in decomposition of the carbamate, after which excess quantities of acid are optionally neutralized with bases and the polyamines are subsequently isolated.
It has now surprisingly been found that aromatic and/or aliphatic polyamines can be obtained by reacting compounds containing free NCO-groups (with NCO-content of from 0.5 to 40%, by weight), with strong bases (aqueous solutions of alkali- and earthalkali hydroxides, alkalisilicates, alkali aluminates or tetraalkylammonium hydroxides) and carefully recovering the polyamines from the resulting compounds containing carbamate groups without additional reaction stages involving further reactants, such as acids to decompose the carbamates. A thermal decomposition and/or solvent extraction are sufficient to recover the desired polyamines from the reaction mixture comprising compounds containing carbamate groups.
Compared with conventional processes, the process according to the invention has several significant advantages.
1. The conversion of the NCO-compounds into amines is carried out simply and economically as a one-pot process.
2. Conventional reaction vessels may be used without any need for safety precautions to be incorporated in their design.
3. The process may also be advantageously carried out continuously.
4. The components on which the NCO-compounds are based (i.e., for example, modified polyisocyanates, polyisocyanates, polyols and amines) are inexpensive products available in large quantities.
5. The reaction yields are substantial and the volume/time yield is high.
6. The process is environmentally sound in that it may be carried out in the absence of solvents, only small quantities of carbon dioxide are released and only very small quantities of salt (carbonic acid salts from the bases) accumulate.